4 star heroes from this event are imho meh. 5 stars are great (except Karnov who is very niche).
Question is - do you feel lucky enough to pull a 5 star hero. With 15 pulls the odds are against you, but you know, it happens. No one who gives you the advices can predict that for you.
If I were you, I would save them. We were told to expect more new events these year - perhaps they will come with better mix of 4s and 5s.